The present invention relates to the supporting of mine roofs and, in particular, to a cutting insert for use in a drill bit for cutting holes in a mine roof.
During mining operations, the roof of the mine must be supported. This has traditionally been accomplished by bolting support plates to the roof, the bolts being installed in pre-drilled holes in the mine roof.
It has been conventional to drill the bolt-receiving holes by means of a drill bit on which is mounted a cutting insert. The cutting insert, formed of a hard material such as cemented carbide for example, is mounted, e.g., by brazing, in a slot formed in a bit body, as depicted for example in U.S. Pat. No. 4,492,278. A conventional insert 10, depicted herein in FIGS. 1-4, includes a pair of linear cutting edges 12, 14 situated on opposite sides of a front face 15 of the insert. Those cutting edges are joined at the center of the insert by a linear central portion or chisel edge 16 which divides the front face into first and second sections 18, 20. Each section of the front face, and thus each of the cutting edges 12, 14, extends laterally outwardly and longitudinally rearwardly with reference to the axis of rotation A of the bit body B.
When the bit body B is rotated, the cutting edges 12, 14 and the chisel edge 16 perform a cutting action. The side faces 22, 24 of the insert serve as chip faces for the cutting edges 12, 14, respectively, and the front face sections 18, 20 serve as chip faces for the chisel edge 16. The angle .alpha. (see FIG. 3) included between the faces 18 and 22 (and thus between faces 20 and 24) is about 70 degrees, whereas the angle .beta. (FIG. 4) included between the faces 18 and 20 at the chisel 16 is about 140 degrees. Due to the relatively large angle .beta., the chisel edge 16 is relatively blunt and unable to perform a significant cutting action; rather, it performs a crushing action on the material forming the mine roof as the insert is longitudinally advanced.
Since it is necessary to drill many holes in the roof of a mine in order that sufficient support can be established, it is desirable that the bit be able to drill each hole as rapidly as possible. Also, it is desirable to maximize the toughness of the insert so that the insert lasts longer and thereby reduces the drilling costs.